Magnetic field sensors generally include a magnetic field sensing element and other electronic components. Magnetic field sensors provide an electrical signal representative of a magnetic field sensed by the magnetic field sensing element. Various types of magnetic field sensing elements are known, including Hall Effect elements and magnetoresistance elements.
Magnetic field sensors provide information about a sensed ferromagnetic object by sensing fluctuations of a sensed magnetic field. Some magnetic field sensors sense a magnetic field associated with a magnet as an object moves within a magnetic field generated by the magnet. In the presence of a moving ferromagnetic object, the magnetic field signal sensed by the magnetic field sensor varies in accordance with a shape or profile of the moving ferromagnetic object (a “target object”).
In automotive applications, a typical magnetic field sensor might determine the position of a target object, for example, the position of a gear shift lever in an automotive transmission. Such systems might beneficially sense a neutral position of a gear shift lever used in a vehicle transmission. For example, stop-start systems reduce fuel consumption and emissions by turning off a vehicle's engine when the vehicle is stopped. Using a neutral gear sensor to sense the neutral position, an engine control unit (ECU) might turn off the engine to reduce fuel consumption when the vehicle is stopped, for example, if a gear shift level is in a neutral position and the clutch is engaged (e.g., the clutch pedal is not pushed). Thereafter, when the clutch is disengaged (e.g., the clutch pedal is pushed to select a gear), the engine control unit starts the engine.
Some current neutral gear sensors employ a single Hall plate, a zero Gauss magnet (i.e., a magnet with an area proximate to the magnet at which the magnetic field is zero) and a ferromagnetic target object. The target object moves in a rotation and a translation with respect to the neutral gear sensor. The air gap between the target and the sensor typically is unchanged along the translation, but varies with the target rotation (e.g., the air gap between the ferromagnetic target object and the sensor changes with the rotation of the target object). This air gap variation corresponds to a variation of the magnetic field measured by the sensor, which can, in turn, be used to determine the gear shift lever position.
However, current neutral gear sensors can be sensitive to external magnetic fields and magnetic field perturbations, decreasing measurement accuracy. Further, since the measurement is unipolar (e.g., the sensed magnetic field is in the same direction regardless of target position), the measurement has high sensitivity to air gap variation (e.g., due to mounting and orientation in different vehicles, manufacturing tolerances, etc.). Conventional systems can also be sensitive to magnetic field strength drift over temperature variations. Consequently, some magnetic field sensors employ continuous time calibration to maintain accurate detection of the neutral position, but such calibration increases system cost and complexity and, in some instances, eliminates the ability to correctly detect additional gear shift lever position(s) (e.g., reverse, etc.). Further, zero Gauss magnets, can increase the overall system cost.
Therefore, it is desirable to provide a system that senses the neutral position as well as other positions of the gear shift lever without requiring continuous calibration and expensive magnetic components.